The invention relates to an ion conductor for electrochemical cells of the generic type such as known from European patent document EP 411 156 A1.
European patent document EP 411 156 A1 discusses an ion conductor which comprises polyphosphazene. The polyphosphazene is formed from phosphazene base units which have organic substituents on the phosphorous of a phosphazene base unit. These polyphosphazenes, however, exhibit inadequate chemical stability with respect to the ions of some alkali metal salts, especially lithium, so that their suitability for use in such batteries is quite limited. Moreover, they decompose at elevated temperatures.
The object of the invention is to provide an ion conductor (as classified) of an electrochemical cell which is chemically more stable with respect to ions of alkali metal salts, which under normal conditions, has as low a melting temperature as possible and which decomposes at temperatures that are as high as possible. A further object of the invention is to provide a method for preparing such an ion conductor.
These and other objects and advantages are achieved by the ion conductor according to the invention in which alkali metal salts are mixed with oligomers and/or polymers containing at least one inorganically substituted phosphazene base unit. As a result of this combination, the melting temperature or glass transition temperature Tg of the alkali metal salts of the ion conductor is lowered to the room temperature range and even lower. The ion conductivity of the alkali metal salts of such ion conductors is up to 1000 times greater at room temperature (T.sub.R =20.degree. C.) than the ion conductivity of the previously known alkali metal salts whose melting point is usually above 100.degree. C.
This makes it possible for an electrochemical cell comprising the ion conductor according to the invention to be operated, without additional energy, at temperatures as low as room temperature and even lower. During operation, the ion conductor heats up owing to the internal resistance of the electrochemical cell, and consequently its temperature rises. This self-heating of the ion conductor of the electrochemical cell, which may lead to temperatures of the ion conductor and its components of far above 150.degree. C., raises ion conductivity of the alkali metal salts in a known manner.
To ensure reliability of the electrochemical cell, polymers are selected which have phosphazene base units that are chemically stable with respect to the alkali metal salts and have high thermal stability. Of particular advantage in this context are polymers having phosphazene base units, which comprise halogens as side chains on the phosphorous of the phosphazene base unit. Preference in this case is given to dihalophosphazenes. These polyphosphazenes comprising halogens in the side chains, which are known as inorganic polymers in the literature, are thermally stable up to approximately 350.degree. C., and their oligomers comprising from three to seven phosphazene base units are thermally stable even up to above 400.degree. C.; they are therefore particularly suitable for use in traction batteries. In particular, the ion conductor according to the invention is suitable as an electrolyte for electrochemical cells comprising alkali metal-containing active materials such as lithium- and sodium-containing active materials. In some cases, however, thermal stability of the polymer from temperatures as low as 150.degree. C. upward may be appropriate.
The ion conductor according to the invention advantageously contains alkali metal salts preferably comprising more than 3% by weight of the final mixture, in particular between 20% by weight and 60% by weight. In the process according to the invention, these are heated above their melting point and fused. The alkali metal salt melt is admixed with the polymers dissolving in the alkali metal salt melt, and mixing with the alkali metal salts. The requirements with respect to the polymer are in this case that it is chemically stable with respect to the alkali metal salts and that it contains at least one conjugated phosphazene base unit, at whose phosphorous atom at least one inorganic atom or an inorganic compound is positioned.
Owing to their low melting temperature and their high ion conductivity, especially with respect to alkali metal ions, at room temperature, the ion conductors according to the invention can potentially be widely used in alkali metal batteries
in which molten salts are used as the electrolyte, e.g. Na/NiCl.sub.2 or AlLi/FeS.sub.2 batteries; PA1 in which electrode reactions proceed as intercalation reactions; PA1 e.g. in alloys within a single-phase domain for the negative electrode and in the form of intercalation compounds for the positive electrode; PA1 in which gel- or paste-like electrolytes are used which are produced by demobilization of the electrolyte by means of inert and/or ion-conducting solid additives and in which solid or liquid electrodes are employed; and PA1 in which gel- or paste-like alkali metals are used which are produced by demobilization of the electrolyte by means of inert and/or ion-conducting solid additives and whose electrodes are arranged as solids in a bipolar sandwich construction.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.